1. Field of the Invention
Embodiments of the invention relate to a semiconductor device and a method of manufacturing a semiconductor device.
2. Description of the Related Art
Silicon (Si) has been used as a constituent material of power semiconductor devices that control high voltage and/or large current. There are several types of power semiconductor devices such as bipolar transistors, insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), etc. These devices are selectively used according to an intended purpose.
For example, bipolar transistors and IGBTs have high current density compared to MOSFETs, and can be adapted for large current but cannot be switched at high speeds. In particular, the limit of switching frequency is about several kHz for bipolar transistors and about several tens of kHz for IGBTs. On the other hand, power MOSFETs have low current density compared to bipolar transistors and IGBTs, and are difficult to adapt for large current but can be switched at high speeds up to about several MHz.
There is a strong demand in the market for a large-current, high-speed power semiconductor device. Thus, IGBTs and power MOSFETs have been intensively developed and improved, and the performance of power devices has substantially reached the theoretical limit determined by the material. Therefore, in terms of power semiconductor devices, semiconductor materials replacing silicon have been investigated and silicon carbide (SiC) has been focused on as a semiconductor material enabling production (manufacture) of a next-generation power semiconductor device having low ON voltage, high-speed characteristics, and high-temperature characteristics (see, for example, K. Shenai, et al, “Optimum Semiconductors for High-Power Electronics”, IEEE Transactions on Electron Devices, September 1989, Vol. 36, No. 9, pp. 1811-1823).
Silicon carbide is chemically a very stable semiconductor material, has a wide bandgap of 3 eV, and can be used very stably as a semiconductor even at high temperatures. Silicon carbide has a critical electric field strength that is ten times that of silicon or greater, and is expected to be a semiconductor material that can sufficiently reduce ON-resistance. These merits of silicon carbide are common to other semiconductor materials having a bandgap greater than silicon (hereinafter, wide bandgap semiconductor material), such as gallium nitride (GaN). Thus, a high-voltage semiconductor device having low resistance can be achieved by using a wide bandgap semiconductor material (see, for example, B. Jayant Baliga, “Silicon Carbide Power Devices”, USA, World Scientific Publishing Co., 2006 Mar. 30, p. 61).
Another semiconductor device realizing low resistance and high breakdown voltage has be proposed in which a portion (base region) forming a channel (inversion layer) is formed by epitaxial growth, whereby a crystalline property of the portion forming the channel is improved and high quality is facilitated (for example, refer to Japanese Laid-Open Patent Publication No. 2006-147789). In Japanese Laid-Open Patent Publication No. 2006-147789, low resistance and high quality are realized by increasing the crystalline property of the portion forming the channel and reducing the channel resistance.
As a semiconductor device in which a portion forming a channel is formed by epitaxial growth, a device has been proposed in which a p-type region is provided at a step that results near an interface of an active region and a termination structure region when in an epitaxial layer that includes the portion forming the channel, a portion in the termination structure region is removed. Thus, distribution of a p-type impurity at the step is gradual in a depth direction, electric field concentration at the step is mitigated, and decreases in breakdown voltage are prevented (for example, refer to Japanese Laid-Open Patent Publication No. 2010-045388).